Classifications

• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
  • H02H9/00—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection
  • H02H9/001—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection limiting speed of change of electric quantities, e.g. soft switching on or off
  • H02H9/002—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection limiting speed of change of electric quantities, e.g. soft switching on or off limiting inrush current on switching on of inductive loads subjected to remanence, e.g. transformers
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F27/00—Details of transformers or inductances, in general
  • H01F27/42—Circuits specially adapted for the purpose of modifying, or compensating for, electric characteristics of transformers, reactors, or choke coils
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
  • H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
  • H02H7/04—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for transformers

Definitions

• the present invention generally relates to the field of electrical engineering. More particularly, the invention relates to devices and methods for reducing the magnitude of inrush current resulting from a power transformer energization by controlling the operations of a three-phase circuit breaker (CB) with simultaneous pole operation, also known as three-pole operated circuit breaker or gang operated circuit breaker.
• CB three-phase circuit breaker
• This patent also applies to independently-operated pole circuit breaker with closing all three poles at the same moment.
• the present invention also relates to controlled switching devices (CSDs) and methods for using the same.
• Electric power systems typically comprise several power transformers as apparatus.
• Such power transformers can be either used as step-up or step-down transformers, which mean they are used to either increase or reduce the voltage level of an electrical circuit.
• Power transformers are generally used by electric utilities in generation, transmission and distribution applications as well as by large energy consumers such as, but not limited to, industries, oil and gas, railway operators, large buildings and facilities.
• a residual magnetic flux may remain in the core of the power transformer. It is generally well known that due to that residual magnetic flux, the uncontrolled energization of a transformer may cause inrush currents having several orders of magnitudes of the rated current value of the transformer. Such event may increase or cause stress on electrical apparatus including, but not limited to, power transformers and associated circuit breakers. Induced stress may lead to premature wear of such equipment and may affect the reliability of the power system or plants and potentially lead to major blackout or plant downtime.
• This paper describes a method for controlling the closing of a circuit breaker at a precise electrical angle calculated based on the magnitude and polarity of the residual magnetic flux of the transformer.
• Such paper also demonstrated that using CB with simultaneous pole operation may mitigate the inrush current at the closing moment of the CB in relation to the residual magnetic flux.
• Such method may be achieved by simultaneously closing the three phases of the CB at an optimum point as a function of the residual magnetic flux pattern.
• This technique requires that the residual magnetic flux in all three phases are known and that the residual magnetic flux magnitudes of two phases are higher than a certain threshold and follow the typical residual magnetic flux pattern (+r, -r, 0). Therefore, this technique limits the scope of use of such method.
• US Patent no. US 8,310,106 discloses a method to mitigate inrush current for power transformer being energized through a simultaneous closing of three-phase circuit breaker or a non-phase segregated operation-type circuit breaker, without providing the circuit breaker with a resistor or other equipment.
• Such method of US Patent no. US 8,310, 106 defines an area of interest where all the three phases must be energized simultaneously to mitigate the inrush current. This area of interest is defined as the region in which the polarities of the steady-state magnetic flux and the residual magnetic flux coincide for all three phases.
• this said method defines that one of the points to be targeted inside this area of interest is the intersection of the steady-state magnetic flux and the residual magnetic flux for the phase with the smallest residual magnetic flux among the phases of the three-phase transformer. Also, this same method defines another point on which the simultaneous closing of the three phases should occur. This point is the peak value of the steady-state magnetic flux of the phase that is having the largest residual magnetic flux.
• This method has limitations since there exist cases where the polarities of the steady-state magnetic flux and the residual magnetic flux can't coincide in the same region for all three phases. For example, in case no delta connection is present on power transformer, the sum of the three residual magnetic fluxes is not 0 all the time and they are all having the same polarity.
• This invention relates to devices and methods achieving controlled switching of power transformers (5) by means of controlling the closing of a circuit breaker (3) with simultaneous pole operation or independent pole operation (with all three phases operated at the same time) with the objective of mitigating the inrush current resulting from transformers energization. More specifically, such devices and methods determine the optimal electrical switching angle of the circuit breaker (3) according to the residual magnetic fluxes of each phases in the transformer's core resulting from its previous de-energization. The residual magnetic fluxes from each phase are calculated from the mathematical integral of the transformer voltage. The transformer voltages are measured using instrument transformers (6 or optionally 8).
• the controlled switching device After having calculated each of the residual magnetic fluxes, the controlled switching device will determine the circuit breaker's closing angle so that it corresponds to the phase with the minimum residual magnetic flux. Thanks to this approach, the controlled switching device is able to close the circuit breaker at an optimum electrical angle, then achieving inrush current mitigation during transformer's energization.
• the present invention discloses a method for lowering the inrush current resulting from a power transformer (5) energization in an electrical circuit comprising a three-phase circuit breaker with simultaneous pole operation (3) or independent pole operation (with all three phases operated at the same time).
• the method comprises the step of providing the electrical circuit with a controlled switching device (7) adapted to send a close command to a circuit breaker (3) in order to synchronize its mechanical operation to reduce inrush current.
• the CSD is connected to the potential transformers on the primary side of the transformer (6) or optionally on the secondary side (8).
• the said method allows controlled switching of a power transformer.
• the method allows the reclosing of said circuit breaker at an optimal electrical angle to mitigate inrush current.
• the present disclosure may be applied to any type of connections of transformers, which can be, but not limited to, ⁇ - ⁇ , Y-Y, ⁇ - ⁇ and ⁇ - ⁇ with either floating or grounded neutral, and any vector groups.
• the method disclosed herein further comprises the step of adjusting the CSD to close the CB of the electrical circuit at an optimal electrical angle to reduce the inrush current resulting from the energization of a power transformer.
• the controlled switching device is connected to any unloaded power transformer.
• the method then further comprises the step of calculating the residual magnetic fluxes inside the transformer's magnetic core by measuring the voltage on the primary or secondary side of the transformer in order to calculate the optimal electrical closing angle of the CB.
• the method disclosed herein comprises the step of reducing the inrush current produced by the energization of the power transformer controlled by a three-phase CB with simultaneous pole operation or independent pole operation (with all three phases operated at the same time for a certain matter) while knowing the transformer's residual magnetic flux.
• the present invention uses a CSD to control a power transformer energization by closing a three-phase CB with simultaneous pole operation or independent pole operation (with all three phases operated at the same time for a certain matter) of an electric circuit at an optimal electrical angle to reduce an inrush current provided to the electrical circuit.
• the said inrush current reduction is achieved through the use of a controlled switching device, alternatively known as a point on wave (POW) controller.
• POW point on wave
• a transformer is de-energized such as by receiving a protection trip signal or a voluntary command or by having a voltage loss in the power system, a residual magnetic flux may remain in each phase of the transformer's magnetic core. To optimize the inrush current mitigation on the next energization, this said residual magnetic fluxes must be taken into account to calculate the optimal electrical closing angle on which to close the CB.
• the present disclosure provides that each time the transformer is de-energized, the voltage is measured either at the high voltage or low voltage side of the transformer, then the residual magnetic flux is calculated, and then the information is processed by the controlled switching device. Furthermore, with the residual magnetic flux information, the controlled switching device will be able to determine the optimal closing angle for the future transformer energization, hence reducing optimally the inrush current.
• a method allowing optimal inrush current mitigation from a transformer's energization controlled by a three-phase circuit breaker with simultaneous pole operation or independent pole operation (with all three phases operated at the same time for a certain matter) of an electrical circuit comprises a circuit breaker (simultaneous pole operation or independent pole operation (with all three phases operated at the same time for a certain matter)), an unloaded power transformer and being electrically fed with an electrical current, the method comprising the steps of providing the electrical circuit with a controlled switching device adapted to send an open or close command to the circuit breaker taking into account the transformer's residual magnetic flux in order to reduce an inrush current sent to the electrical circuit.
• the said method may further use the step of determining the Minimum Flux Simultaneous Closing Strategy (MFSCS) to energize the transformer.
• MFS Minimum Flux Simultaneous Closing Strategy
• the method may further comprise automatically adjusting the electrical switching angle of the circuit breaker based on a further step to calculate all three residual magnetic fluxes in the transformer's core.
• a CSD in another object of the present invention, the use of a CSD in a circuit comprising a three-phase circuit breaker with simultaneous pole operation or independent pole operation (with all three phases operated at the same time for a certain matter) and a power transformer is disclosed, wherein the CSD is adapted to send a close command to the CB to control the transformer's energization in order to synchronize a close mechanical operation of the circuit breaker to reduce an inrush current sent to an electrical circuit comprising the controlled switching device and at least one transformer.
• Figure 1 graphically illustrates an example of inrush current as a function of time (Prior Art).
• Figure 2 illustrates a typical circuit with a power transformer controlled using a three-pole circuit breaker with simultaneous pole operation comprising a controlled switching device.
• Figure 3 graphically illustrates an example showing the relation between the 3- phase voltage signal and the related flux inside the transformer.
• Figure 4 graphically illustrates an example showing the optimum closing angle for residual magnetic flux corresponding to a flux pattern of (+0.25, -0.75, +0.5) respectively for phase A, B, and C.
• This invention relates to the switching of power transformers by controlling the operations of a CB to mitigate and/or eliminate the inrush current using a CSD, wherein the CSD automatically determines the optimal electrical switching angle of the CB according to the magnitude and the polarity of residual magnetic flux in the core of the transformer.
• the CSD evaluates the magnetic residual magnetic flux by means of mathematical integration of the measured voltage at the transformer primary or secondary side when the transformer is de-energized.
• FIG. 2 an example of an electrical circuit connected to a transformer (5) connected to the voltage source (1) using a circuit-breaker (3) on its primary side is shown.
• Such an installation typically comprises a CSD (7) and instrument transformers (6 or optionally 8).
• the CB (3) controls the energization of the transformer (5).
• the present disclosure provides a new method to energize an unloaded power transformer at an optimum angle in order to mitigate/reduce the inrush current.
• a CSD runs computer program implementing an algorithm mitigating the inrush current using the level of residual magnetic flux of all the 3 phases of a power transformer.
• the power transformer may be either a 3 single-phase transformers or a 3-phase power transformer. Such method is applied to CB with simultaneous pole operation, which energizes the power transformer.
• the method comprises calculating the residual magnetic flux of each transformer phase resulting from the de-energization of the power transformer using mathematical integral of the transformer voltage measured on its primary or secondary side, as shown in Figure 2.
• the CSD calculates the prospective flux to be present in the transformer in an energized state.
• the power transformer is energized by simultaneously closing all the CB poles at an angle which produces a prospective flux on the phase having the minimum absolute residual magnetic flux and having the lowest matching closing angle dispersion in reference to the 2 other phases.
• FIG. 4 an example of the optimum closing angle for residual magnetic flux corresponding to +0.25 p.u. (40), -0.75 p.u. (41), +0.5 p.u. (42) respectively for phases A, B, and C, is illustrated.
• the closing angle should be selected according to the phase having the minimum absolute residual magnetic flux magnitude.
• phase A since phase A has a residual magnetic flux of a 0.25 p.u. (40), it shall be selected.
• only 2 solutions corresponding to the phase A minimum residual magnetic flux exist ((40) and (43)).
• a first solution (46) (closing all the three phases at (40))
• the residual magnetic fluxes of phase B (41) and phase C (42) have a minimal flux difference between the prospective magnetic fluxes and their respective residual magnetic fluxes when the transformer is energized.
• the second solution (47) there is a 100 degrees closing angle dispersion between the prospective flux on phase B (48) and its residual magnetic flux (44) when the transformer is energized. Since the maximum closing angle dispersion in the first solution (46) is inferior to the maximum closing angle in the second solution (47), the second solution (47) should be discarded, in order to attain the optimum closing angle for the CB. Since the dispersion of the closing angle of the first solution (46) is inferior, such solution shall be selected for the optimum closing angle of the CB.
• a CSD monitors the voltage on the primary or secondary side of the transformer in order to calculate the value of the residual magnetic flux in each phase calculated as follow:

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• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• Protection Of Transformers (AREA)
• Driving Mechanisms And Operating Circuits Of Arc-Extinguishing High-Tension Switches (AREA)
• Ac-Ac Conversion (AREA)

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• 2015
  • 2015-07-02 EP EP15815351.0A patent/EP3164922A4/de not_active Withdrawn
  • 2015-07-02 US US15/323,424 patent/US10230234B2/en active Active
  • 2015-07-02 CN CN201580039669.4A patent/CN106605344B/zh active Active
  • 2015-07-02 CA CA2922990A patent/CA2922990C/en active Active
  • 2015-07-02 WO PCT/CA2015/050619 patent/WO2016000080A1/en active Application Filing

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